Synthesis of Black TiOx Nanoparticles by Mg Reduction of TiO2 Nanocrystals and their Application for Solar Water Evaporation

TiOx(x < 2) nanoparticles with tunable colors from white to gray to blue-gray to black are synthesized by magnesium (Mg) reduction of white P25 TiO2 nanocrystals followed by removal of excess Mg with aqueous HCl and distilled water. Increasing amounts of Mg smoothly decrease the oxygen content in TiOx which is responsible for the gradual increase in light absorption and concomitant darkening of its color from white to black with decreasing values of x. The as-synthesized TiOx nanoparticles are spin-coated onto the surface of a stainless steel mesh followed by surface superhydrophobization in order to test their performance as a solar water evaporator. Results from the tests show that the black TiOx efficiently generates water vapor with a solar thermal conversion efficiency as high as 50% under solar-simulated light irradiance at an intensity of 1000 W m(-2) (1 Sun). Moreover, TiOx nanoparticles have inherent advantages over other materials used for solar water desalination, such as their tunable light absorption, low-cost, low-toxicity, superhydrophobicity, and chemical stability.